Fuel timing control

ABSTRACT

The invention relates to a spark ignited engine in which charge stratification occurs within the combustion chamber and in which the burn quality of the charge is dependent upon the timing of the instant of introduction of the fuel into the charge. A sensor is provided to produce a signal representative of the burn quality and the fueling timing is controlled in dependence upon the output signal of the sensor so as to optimize the charge stratification within the combustion chamber.

BACKGROUND OF THE INVENTION

The present invention relates to the timing of the instant ofintroduction of fuel into the combustion chamber of a spark ignitedinternal combustion engine in which the charge is stratified.

The concept of a stratified charge is known from many engineapplications as will be described below. The basis of operation is thatthe overall mixture is too lean to burn correctly but by ensuring anuneven distribution of the fuel in the combustion chamber immediatelybefore the spark, it is possible provide a richer mixture in theimmediate vicinity of the spark plug electrodes to guarantee properignition, the powerful flame then being able to propagate and ignite theremaining charge which becomes progressively leaner.

One application in which a stratified charge is required is when thequantity of fuel is being used instead of the intake air volume toregulate the engine power output. Here, when operating under part load,there will be excess air and consequently it is essential in view of theoverall weakness of the mixture to resort to a stratified charge ifproper ignition is to be achieved. This is just one example of a leanburn and others will also come to mind.

Charge stratification also occurs unintentionally when fuel is injectedeither directly into the cylinder or into the intake port while theinlet valve is open. The fuel injection creates a local concentration ofthe fuel in the combustion chamber and its timing has been found to becritical. Good performance is achieved if the rich mixture lies withinthe vicinity of the spark plug electrodes at ignition but not otherwise.Here the charge is meant to be homogeneous but the importance of theinjection timing suggests otherwise.

In the case of injection into the intake port, the best performance isproduced if injection occurs in a narrow time window while the inletvalve is open but the timing is so critical and the performance is sodegraded outside this window that it is generally preferred to time theinjection to take place while the intake valve is closed. Here, theefficiency is not optimised but the injection timing is not critical.This ensures a homogeneous mixture but the efficiency is maximised if aweak mixture is burnt while the charge is correctly stratified.

A still further application in which charge stratification is met is inthe case of two-stroke engines. A two stroke engine running under partload conditions must resort to charge stratification because the mass ofthe trapped gas can only be controlled within narrow limits and theregulation of the power is carried out by limiting the fuel supply.

It has been proposed in the case of a two stroke to provide in additionto the transfer passage and the exhaust port a valve controlled fuellingport in which fuel is accumulated during the engine cycle, the fuelbeing introduced into the combustion chamber at a time when it will notfind its way to the exhaust port. As in the case of direct injectioninto the cylinder of a four stroke engine, the timing of suchintroduction of the fuel into the combustion chamber is important ifadvantage is to be taken of the charge stratification to maximise theengine efficiency.

BRIEF DESCRIPTION OF THE DRAWING

The invention described herein will be more fully understood by readingan example of an embodiment which utilized the invention to advantage,referred to as the preferred embodiment, with reference to the singledrawing wherein:

FIG. 1 is a flowchart of various process steps performed in controllingconventional fuel injection of a convention internal combustion engine.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, there is provided a spark ignitedinternal combustion engine, in which charge stratification occurs withinthe combustion chamber and in which the burn quality of the charge isdependent upon the timing of the instant of introduction of the fuelinto the charge, characterised in that a sensor is provided to produce asignal representative of the burn quality and in that the fuelintroduction timing is controlled in dependence upon the output signalof the sensor so as to optimise the charge stratification within thecombustion chamber.

It is known that spark timing and mixture strength also affect the burnquality of a charge and sensors are already known, such as ionisationsensors, pressure sensors and optical sensors, for feeding back a signalto the ignition system or the fuelling system to maximise burningefficiency. Any such sensor may be used in the present invention forfeedback control of the timing of the fuel introduction.

The burn quality will be maximised at one value of fuel timing and toeach side of this local peak, the burn quality will be degraded withchanges in timing. The feedback signal must act in a direction to returnthe timing to its optimum but additional information is required todetermine the sense of the deviation from the optimum timing.

To mitigate this problem, it is possible for two cylinders (shown asCYL_(i) and CYL_(i+l) in FIG. 1) to be fuelled with a given phasedifference between them as shown by process step 12 in FIG. 1. In thiscase the cylinder should at the optimum setting straddle the efficiencypeak, as shown by process steps 15, 16, 18 and 20 in FIG. 1, and thesense of any deviation can be determined by subtracting the signals ofthe two cylinder sensors from one another, as shown by process step 14in FIG. 1.

Alternative approaches are possible for determining the sense ofdeviation for example long term trends of the corrections applied may bemonitored to determine if the feedback control is aggravating an errorinstead of correcting it or the same cylinder may have its timingdithered between cycles to obtain analogous data.

Various fuelling and spark timing systems have been proposed in the pastwhich rely on a feedback signal from the combustion chamber. The presentinvention can utilise similar control systems t control the timing ofthe introduction of fuel into the combustion chamber and because of theanalogy with the known systems, it is believed that the invention willbe clear to the person skilled in art from the foregoing commentswithout the need for more detailed description.

I claim:
 1. A spark ignited internal combustion engine, in which chargestratification occurs within the combustion chamber and in which theburn quality of the charge is dependent upon the timing of the instantof introduction of the fuel into the charge, comprising:a sensorprovided for each cylinder to produce a signal representative of burnquality in each cylinder; timing means for controlling the fuelintroduction timing in dependence upon the output signal of each sensorso as to optimize the charge stratification within the combustionchamber; and means for indicating a sense of deviation of the fuelingtiming from its optimum setting by fueling two of the cylinders of theengine with a given phase difference between them and subtracting thesignals from the individual sensors associated with the cylinders fromone another to indicate the sense of deviation of the fueling timingfrom its optimum setting and adjusting the fueling timing in dependenceupon the sense of deviation.
 2. An engine as claimed in claim 1, whereinthe sensor for producing a signal indicative of burn quality is anionisation sensor, a pressure sensor or an optical sensor disposedwithin the combustion chamber.